1. Field of the Invention
The present invention relates to object detecting systems, and more particularly, to an object detecting system that detects an object by stereo matching of a pair of images taken by stereo-image taking means.
2. Description of the Related Art
In a typical method for measuring the distance to an object around a vehicle with a stereo-image taking means mounted inside a front glass of the vehicle, a pair of images are taken by a pair of cameras, and one of the taken images used for reference (hereinafter referred to as a reference image) is compared with the other image (hereinafter referred to as a comparative image). By comparison, a difference between positions of corresponding portions of the same object in the images, that is, a parallax is calculated, and the distance to the object is calculated from the parallax according to the principle of triangulation. The corresponding portions of the same object in the reference image and the comparative image are typically located by stereo matching (for example, see Japanese Unexamined Patent Application Publication No. 10-283461).
In such stereo matching, for example, when an image of an object is taken by only one of a pair of cameras since a wiper passes over the front glass in front of the other camera and the other camera is hidden by the wiper, it is difficult to specify the position of the object in the image taken by one camera, and to calculate the distance to the object.
Japanese Unexamined Patent Application Publication No. 2000-123151 proposes an image processing method using a single camera. In this method, differences in pixel density between an image of an environment in front of a vehicle taken in the current frame and an image of the environment taken in the previous frame are calculated. On the basis of the average density and distribution of a difference image obtained from the differences, it is determined whether an image of the wiper is included in the image taken in the current frame.
Japanese Unexamined Patent Application Publication No. 2006-107314 also proposes an image processing apparatus using a single camera. In this apparatus, the moving speeds of objects in a taken image are calculated by using optical flow in which the moving directions and speeds of the objects in the image are shown by vectors. On the basis of the moving speeds, a wiper whose image is included in the taken image is detected. When the wiper is detected in the image, an area of the image of the front environment missing because of the wiper is compensated for by a compensation image obtained from the previous frame.
Unfortunately, in a case in which the technique disclosed in Japanese Unexamined Patent Application 2000-123151 is applied to a pair of cameras in the stereo-image taking means, when an external environment rapidly changes between the previous frame and the current frame, for example, when sunbeams stream through leaves or shade is given while a preceding vehicle is traveling along a road lined with trees, it may be erroneously determined that an image of the wiper is included in the image taken in the current frame.
When the technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-107314 is applied to the stereo-image taking means, frames need to be taken at a sufficiently high shutter speed to successively capture the motion of the wiper in order to calculate the moving speed of the wiper. Moreover, a pair of cameras need to be mounted at a distance from the front glass so as to capture the motion of the wiper through several frames. However, in situations other than wiper detection, it is usually unnecessary to take images at a high shutter speed, and it is often difficult to mount the stereo-image taking means at a distance from the front glass, for example, because of layout constraints. In addition, much time is normally taken to calculate the optical flow, and this puts a heavy burden on the operations.
When objects around the vehicle are detected on the basis of an image taken by the stereo-image taking means, the stereo-image taking means is forced to be mounted inside the front glass, and noise inevitably occurs in the taken image because of raindrops and dirt adhering to the front glass. Further, it is necessary to remove raindrops and dirt adhering to the front glass in front of the camera with the wiper while rain is falling, and an image of the wiper is inevitably included in the image taken by the camera. When noise, such as images of the wiper and adherents, is included in the taken image, it is difficult to obtain highly reliable information about the distance to the object obtained by stereo matching performed.
In contrast, in the above-described case in which an external environment rapidly changes between the previous frame and the current frame, for example, when sunbeams stream through leaves or shade is given while the preceding vehicle is traveling along a road lined with trees, as long as an image of the external environment has been properly taken, highly reliable information about the distance to the object can be obtained by stereo matching based on the taken image.
Therefore, an object detecting system that detects objects by stereo matching is required to accurately detect and distinguish between changes of the external environment and noise due to the wiper and adherents.